1. Field of the Invention
The present invention relates to a switching type echo canceler and, more particularly, to a switching type echo canceler for suppressing an echo generated by speech circulation in a handsfree device for a mobile phone, in a handset, or on a line side.
2. Description of the Prior Art
The mechanism of echo generation in a handsfree device will be described below with reference to FIG. 1. As shown in FIG. 1, for a handsfree device in an automobile, a loudspeaker SP is placed close to feet on the passenger seat, and a microphone MIC is attached to the sun visor. When a speech communication is to be performed by using the handsfree device with the above arrangement, loud speech is output from the loudspeaker SP such that the driver can sufficiently hear it. When this speech is input to the microphone MIC, speech circulation occurs to generate an echo (reverberation). Additionally, a sound generated in a predetermined closed space spreads throughout the space and hits a variety of obstructions. Part of the sound is absorbed and attenuated while the remaining is reflected. The reflected sound further hits another obstructions and is repeatedly reflected while being absorbed and attenuated. At this time, the sound repeatedly reflected is partially input to the microphone MIC and transmitted in the transmission system, thereby generating an echo. As described above, an echo generated in an automobile has infinite acoustic coupling (echo path). The components (time delay and magnitude) of the echo are not always constant.
The mechanism of echo generation in a handset HS will be described below with reference to FIG. 2. As is apparent from FIG. 2, echo generation caused by acoustic coupling (speech circulation) in the handset HS is roughly classified into the following four cases:
(1) A speech output from the receiver (loudspeaker) is transmitted through the handset HS and input to the transmitter (microphone). PA1 (2) A speech output from the receiver vibrates the handset HS and is input to the transmitter. PA1 (3) A speech output from the receiver is transmitted through a space and input to the transmitter. PA1 (4) A speech output from the receiver is transmitted through the human body and input to the transmitter.
There is also echo generation on a line side. This will be described below with reference to FIG. 3. As shown in FIG. 3, in a hybrid circuit HYB, transmitter speech V.sub.1 on the four-wire side circulates into receiver speech V.sub.2 (leakage V.sub.3) and returns to the receiver, thereby generating an echo.
A conventional echo canceler for suppressing an echo has an arrangement as shown in FIG. 4. Speech circulation on the line side and acoustic coupling on the handsfree device (HF) side are independently canceled by independently provided echo cancelers 20A and 20B. The echo canceler 20A on the line side comprises a speech sample storing RAM 21 for storing an output signal to the line, a tap coefficient storing RAM 22, a pseudo echo generation unit 25 for receiving data from the speech sample storing RAM 21 and data from the tap coefficient storing RAM 22 to generate a pseudo echo for canceling an output signal circulating from the line side, and a tap coefficient updating unit 27 for updating the tap coefficient in the tap coefficient storing RAM upon reception of data from the speech sample storing RAM 21 and an output signal to the HF side. The echo canceler 20B on the HF side comprises a speech sample storing RAM 23 for storing an output signal on the HF side, a tap coefficient storing RAM 24, a pseudo echo generation unit 26 for receiving data from the speech sample storing RAM 23 and data from the tap coefficient storing RAM 24 to generate a pseudo echo for canceling acoustic coupling (speech circulation) from the loudspeaker on the HF side to the microphone, and a tap coefficient updating unit 28 for updating the tap coefficient in the tap coefficient storing RAM 24 upon reception of data from the speech sample storing RAM 23 and an output signal to the line.
If such a conventional echo canceler of a two-way simultaneous operation type is realized by a digital signal processor (DSP) chip, the arithmetic processing amount becomes twice that of one of the echo cancelers beyond the processing capability of the DSP. Therefore, such a device can hardly be realized by one chip and requires two expensive DSP chips. To manage to realize the device by one chip, the number of tap coefficients must be impractically decreased, resulting in a degradation in performance of the echo canceler. As a result, the expensive DSP chip is dedicated for echo cancel processing, and another signal processing cannot be simultaneously performed. If this function is to be realized by hardware, the circuit scale is doubled.